Pn wave velocity and Moho geometry in north eastern India

Pn velocity has been computed across the NE India and Moho geometry constra ined, using regional earthquake travel times recorded by a network of 30 se ismological stations operated during February-May 1993. Using an appropriat e velocity model and the arrival times at the network stations, preliminary hypocentres of 16 regional earthquakes provided by NEIC were also improved . The average Pn wave velocity in NE India has been found to be 8.5 +/- 0.2 km/s. It varies from 8.3 to 8.5 km/s beneath the Shillong Plateau, Mikhir hills and Assam valley, which is significantly higher than those in other p arts of India. The crustal thickness in NE India is also high, varying from 45-49 km under the Shillong plateau and the adjoining region to 55-65 km i n the convergence zone. The presence of a thick crust and high Pn velocity suggests that the lithosphere in NE India is colder, as also indicated by t he observed deeper level (45-51km) seismicity of the region.